1. Field of the Invention
The present invention relates to thermosettable coating compositions, and more specifically, the present invention relates to thermosettable compositions for powder coatings and the powder coatings made from the thermosettable compositions.
2. Description of Background and Related Art
Thermoset resins, such as epoxy resins (epoxies), having good thermal resistant properties, are desirable for many applications such as liquid coatings, powder coatings, and castings. Glycidyl ethers of aromatic diphenols are a class of epoxy resins that are commonly used, and there are many that are commercially available. Three desirable properties for these aryl glycidyl ethers are high glass transition temperature (Tg greater than 190° C. by dynamic mechanical thermal analysis with dicyandiamide cure), low monomer viscosity (less than 200 mPa-s at 150° C.), and high epoxy equivalent weight (EEW). Epoxies of the present invention exhibit viscosities as low as 120 mPa-s and the dicyandiamide-cured thermosets have Tgs up to 202° C. and EEWs of greater than 190 grams/equivalent (g/eq).
High Tgs are needed for applications where the coating will be exposed to high temperature, for example to coat steel pipe used for transporting hot oil. The properties of the coating degrade precipitously above the temperature of the Tg. Low viscosity resins make it easier to prepare smooth coatings that are free from defects, such as pinholes. Also, adhesion usually is better for low viscosity coatings because wetting of the complex microstructure of the substrate is better. Finally, epoxy resins with high EEWs give thermosets with a relatively low concentration of hydroxyls in the backbone. Hydroxyls are formed during typically curing reactions of epoxy resins, such as with dicyandiamide, a multifunctional amine. There is a direct relationship between hydroxyl concentration in a thermoset and water absorption. High water absorption is undesirable in coatings because corrosion rates are typically higher and longevity in wet or humid environments suffers.
There are many aryl glycidyl ethers that achieve these properties individually, but not that meet them all properties simultaneously. This balance of properties is difficult to achieve. For example, one common strategy for high Tg is to use polyglycidyl ethers of highly functional polyphenols, especially phenol formaldehyde novolacs which are known as epoxy novolacs. However, examples of such novolacs having viscosities of less than about 200 mPa-s are not capable of achieving high Tgs. For example, D.E.N.™ 438 (trademark of The Dow Chemical Company), an industry standard epoxy novolac, has a viscosity of <200 mPa-s but the Tg of the dicyandiamide-cured thermoset is only 173° C.
Accordingly, there is still a need in the coating industry to develop new thermoset resins useful for coatings that are derived from difunctional resins with a balance of properties including high Tg (>190° C.), low monomer viscosity (<150 mPa-s at 150° C.) and high EEW (>190 g/eq).